1. Field of the Invention
This invention relates generally to a method and apparatus for scanning an ablating laser beam to ablate tissue. More particularly, it relates to a method and apparatus for scanning an ablating laser beam to ablate corneal tissue resulting in improved smoothness and uniformity of the remaining tissue.
2. Description of Related Art
The ablation of human tissue with an ablating laser beam is known. Typically, a laser beam is scanned across rows of ablation points in an ablation zone of a layer of tissue to be ablated, and adjacent points or spots in each row are sequentially ablated to reshape the underlying tissue.
For example, laser ablation is utilized to reshape a cornea of an eye to correct refractive disorders. In a normal human eye, the cornea bends or refracts incoming light rays causing light rays to focus on the retina of the eye. Improper refraction of incoming light rays causes blurred vision or a refractive disorder. Myopia (nearsightedness) is one of the most common refractive disorders. In a nearsighted eye, the cornea is too steep, causing light rays to be focused in front of the retina, not at the retina, causing distant objects to appear blurred.
To correct nearsightedness, laser vision correction, e.g., Photorefractive Keratectomy (PRK), can be performed to make the cornea less steep. In PRK, an ablating laser beam removes successive layers of corneal tissue from an ablation zone of a cornea. Each layer of corneal tissue is removed by ablating the corneal tissue with a series of adjacent laser beam pulses disposed generally along a linear path (e.g., a row). By reshaping the cornea to be less steep, light rays focus properly on the retina of the eye, and the nearsightedness condition is corrected.
Conventional laser ablation for vision correction has achieved good results, with the majority of patients no longer being dependent on corrective lenses after the treatment. However, rough or non-uniform tissue remaining after the treatment may cause imperfect results detracting from the full potential of the laser ablation treatment. Thus, the present inventors have recognized that it is important to achieve a smooth and uniform ablation such that remaining tissue does not contain any significant ridges or other rough areas therein.
It is known that the patient's eye moves when undergoing ophthalmic refractive laser surgery. Thus, full treatments are typically performed in a short amount of time, e.g., in under one minute, to minimize the effect of the movement of the eye. Nevertheless, predetermined sequential scans of the ablating laser beam may over ablate some areas and under ablate other areas due to the movement of the eye during the ablation procedure. Therefore, the remaining tissue may not be as smooth or uniform as planned.
FIG. 1 shows a conventional scan pattern across an ablation zone.
In FIG. 1, an ablation layer, defined by ablation zone 135, of a patient's eye that ideally remains stationary during an ablation treatment of an ablating laser beam, is scanned with an ablating laser beam across rows 1 to 10, from left to right. If the eye remains stationary, the ablation points are distributed evenly throughout the ablation zone, and the remaining corneal tissue will be relatively smooth and uniform.
FIG. 2 shows the same ablation zone 135 as in FIG. 1, but as resulting when the patient's eye has moved downwardly at the end of the third scanning line 3. As a result, the fourth and fifth scanning lines 4, 5 as shown in FIG. 2 are scanned on previously ablated areas of the ablation zone 135 instead of at the proper, or planned location as shown in FIG. 1.
Then, before the sixth scanning line 6, the patient's eye moves back to the correct or intended position, and proper or intended ablation continues.
As a result of the eye movements noted above, the corneal surface is ablated too deeply in certain portions thereof (i.e., between the first and third scan lines 1, 3) and is ablated too shallowly in another portion of the cornea (i.e., between the third and sixth scan lines 3, 6). This results in a rough and/or non-uniform surface on the remaining corneal tissue.
The ablation points shown in FIG. 1 and FIG. 2 represent the center of each ablating laser beam pulse. Significant ablation overlap can occur when the ablating laser beam is larger than the distance between each ablation point. However, when the eye moves during corrective eye surgery, the resulting or actual location of the scanned ablation points with respect to the ablation zone 135 is significantly deeper than other properly scanned ablation points, while other portions of the corneal tissue may not have been ablated as intended. This may result in uneven, rough and/or non-smooth ridges on the cornea.
Even if the patient's eye does not move during the ablation procedure, an uneven distribution of ablating laser beam pulses can cause roughness on the cornea.
Accordingly, there exists a need for a method and apparatus for scanning an ablating laser beam which smoothly and uniformly ablates tissue in the face of real-world conditions, e.g., a moving eye.